I used to use an Automator action for this, but Mac OS X Hints unearthed a sweet Snow Leopard shortcut for geeks: the ability to show or hide hidden "dot" files in any Open or Save As dialog.

The magic combination to start imprinting into your fingers' muscle memory is Cmd+Shift+Period. Hit it again to toggle the visibility. Hit the play button above to see what it does in my Mac's home directory. One of the biggest stumbling blocks for Mac users who try out my Todo.txt CLI is finding and opening hidden "dot" configuration file (which is the standard Linux naming convention), so this will help me help them. Sadly this shortcut does not work when you're just browsing in Finder.